Pasty multi-component materials, such as dental impression materials, are often stored separately from each other as individual components, because once the components come into contact with each other a chemical reaction is initiated that eventually turns the mixed composition into a hardened mass. For that reason, such materials are widely available in packages that include two compartments or two separate containers that keep the components initially isolated from each other.
Dental materials as mentioned are generally mixed together shortly prior to use in the dental practice by the dentist or the dentist's assistant. In recent years, devices have been developed which provide for the automatic mixing and dispensing dental impression materials, which provide high precision with regard to the ratio of the two components to be mixed, and the homogeneity of the mixture. The dental impression material components are simultaneously supplied from separate material chambers to a mixer, which mixes and dispenses the mixed paste from a front end. The mixer may be a static mixer (meaning that the structures that cause mixing do not move relative to the mixing chamber) or a dynamic mixer (meaning that the structures that cause mixing do move relative to the mixing chamber, normally in a rotary manner). The paste exiting from the front end of the mixer may be supplied directly onto a dental impression tray that can be placed in a patient's mouth. When the mixed components harden, the tray is removed and the hardened impression material provides an impression of the patient's teeth.
Once the material components have come into contact with each other, normally near or within the mixing chamber, the material in the mixing chamber can only be stored for a relatively short time because the mixed material will soon harden inside the mixing chamber if it is not dispensed and used. Therefore mixers used with such devices generally can only be used once, and are therefore generally exchangeable and often disposable parts. Dentists and their assistants may have to remove and replace mixers several times each day.
Examples of dynamic mixers are found in, e.g., WO 00/21652, EP-A-1 149 627, U.S. Pat. No. 5,249,862 or DE-U-297 05 741. These known dynamic mixers have at their rear end (the inlet side) a central opening for coupling to a drive shaft of a motorized mixing and dispensing device, and two additional inlet connectors for receiving the material components which are to be mixed. The central opening has a hexagonal cross-section for engagement with a corresponding drive shaft also having a hexagonal cross-section, which enables the motor of the mixing and dispensing device to rotate a mixing rotor having mixing paddles.
To ensure proper mixing and dispensing, the mixer has to be coupled correctly to the drive shaft of the device and the connectors for the material components. If the mixer is improperly coupled to the drive shaft, the motor will not be able to drive the mixing rotor correctly, resulting in unmixed or poorly mixed material components. If the connectors for the material components are improperly aligned and connected, material components can escape from the mixer and make a really big mess. With a device for mixing and dispensing two-component materials, a three-way-coupling is thus provided at the device, comprising two material connectors and the drive shaft, which has to be combined with corresponding parts of the mixer. The connectors and the drive shaft are linearly arranged side by side, with the drive shaft located in the middle.
Because the drive shaft, with its hexagonal cross-sectional shape, is oriented by random, coupling of the mixer to the device normally requires a two-step process. First, the user orients the drive-shaft opening of the mixer with respect to the orientation of the drive shaft, and places the mixer at least partly onto the drive shaft without regard to whether the material component connectors are aligned. Then the user aligns the component connectors on the mixer with the corresponding connectors on the device, and finishes connecting the mixer to the device. The drive shaft and the material component connectors are thus all aligned and connected, and the device should operate as expected. However, because the mixer is often attached to the dispenser quickly, the two may not be properly engaged or seated, resulting in unsatisfactory performance.